Original Male Character
Used in fanfiction when an original character is inserted into an already existing set of characters/fandom. Often shunned due to fear of Gary Stus, hence it is given as a warning.
See also: OFC

Pairing: Buffy/OMC
I'm terrified to use my OMC because some might think of him as a Gary Stu.

Oh My Cthulhu - a replacement for "Oh My God" (aka "omg") for those who prefer one (for whatever reason, including you're not religious or you're too religious to feel comfy using "god"). OMC evokes the great chaotic deity from Lovecraftian lore. Often used humorously.